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<H3>fix append/atoms command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID append/atoms face ... keyword value ... 
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command 

<LI>append/atoms = style name of this fix command 

<LI>face = <I>zhi</I> 

<LI>zero or more keyword/value pairs may be appended 

<LI>keyword = <I>basis</I> or <I>size</I> or <I>freq</I> or <I>temp</I> or <I>random</I> or <I>units</I>  

<PRE>  <I>basis</I> values = M itype
    M = which basis atom
    itype = atom type (1-N) to assign to this basis atom
  <I>size</I> args = Lz
    Lz = z size of lattice region appended in a single event(distance units)
  <I>freq</I> args = freq
    freq = the number of timesteps between append events
  <I>temp</I> args = target damp seed extent
    target = target velocity for region immediately ahead of the piston
    damp = damping parameter (time units)
    seed = random number seed for langevin kicks
    extent = extent of thermostated region (distance units)
  <I>random</I> args = xmax ymax zmax seed
    <I>xmax</I>, <I>ymax</I>, <I>zmax</I> = maximum displacement in particular direction (distance units)
    <I>seed</I> = random number seed for random displacement
  <I>units</I> value = <I>lattice</I> or <I>box</I>
    <I>lattice</I> = the wall position is defined in lattice units
    <I>box</I> = the wall position is defined in simulation box units 
</PRE>

</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all append/atoms zhi size 5.0 freq 295 units lattice
fix 4 all append/atoms zhi size 15.0 freq 5 units box
fix A all append/atoms zhi size 1.0 freq 1000 units lattice 
</PRE>
<P><B>Description:</B>
</P>
<P>This fix creates atoms on a lattice, appended on the zhi edge of the
system box.  This can be useful when a shock or wave is propagating
from zlo.  This allows the system to grow with time to accommodate an
expanding wave.  A simulation box must already exist, which is
typically created via the <A HREF = "create_box.html">create_box</A> command.
Before using this command, a lattice must also be defined using the
<A HREF = "lattice.html">lattice</A> command.
</P>
<P>This fix will automatically freeze atoms on the zhi edge of the
system, so that overlaps are avoided when new atoms are appended.
</P>
<P>The <I>basis</I> keyword specifies an atom type that will be assigned to
specific basis atoms as they are created.  See the
<A HREF = "lattice.html">lattice</A> command for specifics on how basis atoms are
defined for the unit cell of the lattice.  By default, all created
atoms are assigned type = 1 unless this keyword specifies differently.
</P>
<P>The <I>size</I> keyword defines the size in z of the chunk of material to
be added.
</P>
<P>The <I>random</I> keyword will give the atoms random displacements around
their lattice points to simulate some initial temperature.
</P>
<P>The <I>temp</I> keyword will cause a region to be thermostated with a
Langevin thermostat on the zhi boundary.  The size of the region is
measured from zhi and is set with the <I>extent</I> argument.
</P>
<P>The <I>units</I> keyword determines the meaning of the distance units used
to define a wall position, but only when a numeric constant is used.
A <I>box</I> value selects standard distance units as defined by the
<A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or metal.
A <I>lattice</I> value means the distance units are in lattice spacings.
The <A HREF = "lattice.html">lattice</A> command must have been previously used to
define the lattice spacings.
</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>.  None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix.  No global or per-atom quantities are stored
by this fix for access by various <A HREF = "Section_howto.html#howto_15">output
commands</A>.  No parameter of this fix can
be used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
</P>
<P><B>Restrictions:</B>
</P>
<P>This fix style is part of the SHOCK package.  It is only enabled if
LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
LAMMPS</A> section for more info.
</P>
<P>The boundary on which atoms are added with append/atoms must be
shrink/minimum.  The opposite boundary may be any boundary type other
than periodic.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_wall_piston.html">fix wall/piston</A> command
</P>
<P><B>Default:</B>
</P>
<P>The keyword defaults are size = 0.0, freq = 0, units = lattice.  All
added atoms are of type 1 unless the basis keyword is used.
</P>
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